1. Field of the Invention
The present invention relates to a method of treating and smoothing a surface of a sliding member and a counter member such as a shim and a cam for driving a valve of an engine.
2. Description of the Related Art
Typically, intake valves and exhaust valves of an engine are driven by cams on camshafts. Such a cam moves keeping in contact with a cam follower such as a tappet at the top of the valve and a shim or a rocker arm attached to the tappet. Since frictional resistance between these cam and cam follower is one of causes of engine output loss, it is preferred to make a sliding surface of the member as smooth like a mirror surface as possible so as to lower its frictional resistance against the counter member. It has been proposed to finish a member that slides keeping in contact with a cam to a sliding surface with a surface roughness of less than 0.3 .mu.mRz (in terms of ten point average roughness) and apply a coating of a material having a hardness higher than 100 Hv such as titanium oxides to the sliding surface by arc discharge ion plating. One of such techniques is known from, for instance, Japanese Unexamined Patent Publication No. 7 - 118832, entitled "Sliding Member and its Production. According to the technique," a sliding surface of a member is formed with droplets of titanium oxide thereon by coating a titanium oxide on titanium stuck on the sliding surface so as to form fine protrusions thereon. While the sliding member slides with its sliding surface kept in contact with a cam surface at the beginning of use, the fine protrusions, on one hand, polish the cam surface to a mirror surface and on the other hand are removed by the cam surface from the sliding member. Another technique that is described in Japanese Unexamined Patent Publication No. 9 - 302454, entitled "Pre-Treatment of Carburizing Quenched Material And Manufacture," a sliding member is polished to a surface with a roughness between approximately 6 and 25 .mu.mRy (in terms of maximum roughness) by shot peening and subsequently treated by carburizing and quenching. By means of this technique, while the sliding member slides with its sliding surface kept in contact with a counter member at the beginning of use, an abnormally carburized surface layer on the sliding member is removed, so as to smooth the sliding surface of the sliding member due to an adaptation effect between the two members.
In the former smoothing technique described in Japanese Unexamined Patent Publication No. 7 - 118832, although control of a current density can govern the droplet density to some extent, it is difficult to realize stable control of the droplet density. If the droplet density is low, it is hard to smooth sufficiently a sliding surface of a counter member, and, conversely, if it is high, scuffing or light seizing is apt to occur in the sliding surface. Because the size, height and position of droplet are hardly controllable, manufacturing variations of the sliding member are great. In addition the arc discharge ionic plating causes an increase in manufacturing costs of the sliding member. On the other hand, the alternative technique described in Japanese Unexamined Patent Publication No. 9 - 302454, while the running-in of a sliding member, which is primarily performed with an attention of removing an abnormally carburized surface layer on the sliding member at the beginning of use, can lower the surface roughness of the sliding member, however, since the surface roughness of the sliding member before the carburizing and quenching treatment is significantly high, the smoothness of a sliding surface that is provided by the alternative technique is limited. Furthermore, it is hard for a sliding surface of the counter member to be smoothed because the alternative technique does not take into consideration the surface roughness of the sliding member.
The term "surface roughness (.mu.mRa)" as used in the specification shall mean and refer to an arithmetic average roughness Ra measured in .mu.m. When taking X-axis and Y-axis in the direction of the average line of a roughness curve sampled by a standard length l, and in the direction of the average line of a roughness curve, respectively, and expressing the roughness curve by a function y=f(x), the arithmetic average roughness Ra is given by the following expression: ##EQU1##